Information about CV

7/27/2019 Information about CV

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Cyclic Voltammetry


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Cyclic Voltammetry

Introduction

Electrochemical analyses can be thought of in terms of two broad classes of

measurement, one in which the potential that develops between two electrodes is measured

(potentiometry) and another in which the current that flows between two electrodes is measured

(amperometry). In potentiometry, it often proves helpful to arrange things such that the current

is very low (e.g., by placing a high-resistance voltmeter in series between two electrodes). The

electrochemical potential of one electrode (the reference electrode) is usually fixed, so the

measured cell potential can be interpreted in terms of an equilibrium half-cell reaction involving

an analyte species in contact with the other electrode (the working electrode). In favorable cases,

one can use data from potentiometric measurements to calculate analyte concentrations directly

from the Nernst equation. Potentiometry is a simple and straightforward analytical method, and

is routinely used to solve many problems in the analysis of electrochemically active and/orcharged analytes.

Figure 1. Schematic diagrams for two-electrode electrochemical experiments.Left; Potentiometry. Right; Amperometry

An important assumption in potentiometry is that the measured potential accurately

reflects the equilibrium position of a well-defined electrochemical cell reaction. Often this is not

the case, however, and potentiometric methods cannot be used. In many situations, it is instead

more appropriate to control the potential of the working electrode (relative to a reference

electrode) and to measure the resulting current. (Recall that current is simply the flow rate of

electrons in a circuit; an ampere of current corresponds to a coulomb of charge flowing per

second.) The magnitude of the resulting current and its dependence on the applied potential then

provide the analytical information. An experiment in which the potential applied to the working

electrode is swept at a constant sweep rate and the resulting current measured as a function of

potential is called a voltammetry experiment, and much of the recent interest in electroanalytical

chemistry stems from the use of voltammetry to obtain analytical (e.g., concentration),

thermodynamic (e.g., redox potentials and equilibrium constants), kinetic (e.g., rate constants for


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reactions involving electrogenerated species) and mechanistic information about chemical

systems in which redox chemistry plays a role.

This laboratory experiment will introduce you to cyclic voltammetry as a simple, rapid,

and powerful method for characterizing the electrochemical behavior of analytes that can be

electrochemically oxidized or reduced. A number of excellent introductory articles on cyclic

voltammetry have appeared, and you are encouraged to consult them for more information on the

basics of voltammetry in general and on cyclic voltammetry in particular1-3

. You will use solid

electrodes made from two different materials, platinum and glassy carbon, in the course of this

experiment. Each material has its own unique advantages and disadvantages. First, you will

measure voltammograms for proton reduction and oxygen reduction at both types of electrodes,

in both acidic (pH = 0) and neutral (pH = 7) aqueous solutions. This will emphasize the

difference between the two materials, and the importance of removing oxygen from solutions to

be analyzed. Second, you will acquire voltammetric data for the electrochemical reduction of the

herbicide N, N'-dimethyl-4,4' bipyridine, commonly known as methyl viologen or paraquat. Theherbicidal action of this molecule derives from its ability to act as an electron acceptor, thereby

disrupting the metabolic electron transport chain in plants, it is therefore important to know what

the redox potential is for reduction of methyl viologen.

Finally, you will explore a strategy for introducing charge selectivity into voltammetric

analyses. Specifically, you will coat an electrode with a thin film of an anionic fluoropolymer

that goes by the trade name Nafion. A thin film of this polymer can be easily prepared by

evaporation of a solution of the polymer in alcohol. Each repeat unit of the polymer has an

anionic sulfonic acid group, and so the film consists of a large number of fixed anionic sites,

each site having in addition its own associated cation. As you will see, the net effect of this

anionic polymer being on the electrode surface is to make cations the only mobile species in the

vicinity of the electrode surface. Anionic species will not partition into the Nafion film and

therefore cannot be transported to the electrode surface. They are therefore not available to beoxidized or reduced, and so we have devised a strategy for discriminating against anionic

interferents in voltammetric measurements.


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The Nernst Equation, Mass Transfer, and Cyclic Voltammetry

It will be helpful to begin with a brief discussion of the basics of amperometry and

voltammetric analysis. You are again encouraged to consult the references for a more thorough

treatment. All electrochemical reactions are governed at least in part by the Nernst equation.

This fundamental expression simply specifies the relationship between the potential of an

electrode and the concentrations of the two species (designated O and R) involved in the redox

reaction at that electrode:

O + n e- R

E=E 0 +RT

nFln

CO

CR

Eo' is the redox potential for the couple involving O and R, C O is the concentration of the

oxidized half of the couple and CR is the concentration of the reduced half. (For example, in

the case of the Fe3+/2+

couple, Fe3+

corresponds to O, and Fe2+ to R.) One could think of

either O or R as the analyte in such an experiment. The concentrations CO and CR in the

Nernst equation apply to the solution immediately adjacent to the surface of the electrode.

In an amperometry experiment, one applies a potential to an electrode, thereby forcing

the ratio (CO/CR) at the electrode surface to adopt a specific value consistent with the Nernst

equation. The concentrations near the electrode surface may or may not be the same as the

corresponding concentrations in the bulk of solution (i.e., far from the electrode surface). If the

concentrations at the electrode surface happen to be the same as those in bulk solution, then there

is no driving force for transport of analyte to or from the electrode surface. (We consider only

the case of transport by diffusion.) The current due to oxidation/reduction of analyte will then be

zero. For a given solution containing both O and R, there can be only one unique potential

where the current is exactly zero.

(where m is small and X + is an exchangeable cation)

The Repeating Unit of Nafion

(CF2CF2)mCF2CF

(OCF2CF)mOCF2CF2SO3-

X+

CF3


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In general, however, the applied potential will be such that CO and CR at the electrode

surface are not the same as those in the bulk of solution. When this is the case, there is a driving

force for transport (diffusion) of analyte to or from the electrode surface. For example, if CO at

the electrode surface in less than CO in bulk solution, then O will be transported from the bulk of

solution to the electrode surface in an attempt to equalize the concentrations. This transport of

analyte to the electrode surface, coupled with the oxidation or reduction of analyte (reduction in

the case of transport of O) as it arrives at the surface, determines the magnitude of the current in

an amperometry experiment. If electron transfer is fast (i.e., the electrode reaction is reversible)

then the current is determined solely by the rate of mass transfer of analyte to the electrode

surface. The difference in concentration between the solution near the electrode surface and that

far from it is in turn determined by the value of the applied potential, via the Nernst equation.

To understand better how potential, concentration, and mass transfer all act to limit the

current, it is helpful to recall some concepts from physical chemistry. Fick's first law states that

material will diffuse from a region of high concentration to one of low concentration. Theresulting flux (recall that flux = moles of material diffusing per unit area per unit time) is given

by

Flux =moles

areatime=

dN

Adt= D

C

x

x=0

where D is the diffusion coefficient for the species being transported to the electrode surface, and

(C/x)x=0 is the partial derivative of concentration of that species with respect to distance,

evaluated at the electrode surface. This partial derivative is simply the slope of a concentration

(C) vs. distance (x) curve, where the surface of the electrode is taken as x = 0. In

electrochemistry, the flux is easily converted into a current by invoking Faraday's Law, which

states that the number of moles of a species undergoing oxidation or reduction is related to the

charge passed for that oxidation or reduction by the Faraday constant:

Q = nFN

Noting that current is simply the flow rate of electrons (i.e., the derivative of charge with respect

to time), it can be easily shown that, for the case of transport of analyte toward the surface, the

following expression holds:

i = dQdt

= nFdNdt

= nFA(Flux) = nFAD Cx

x= 0

This is the fundamental relationship that is used to calculate the current in any amperometric

experiment, including voltammetric experiments. Figure 2 illustrates the points made in the

above discussion. You should be sure that you understand these concepts.


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Figure 2. Concentration vs. distance profiles for species O at electrodes. Left; zero current condition.

Center; reduction. Right; oxidation.

In general, the term (C/x)x=0 is time dependent, and is not trivial to evaluate. It

depends in part on how the applied potential is varied with time, since the potential determines

the ratio (CO/CR) at the electrode surface. The full solution of (C/x)x=0 vs. time, and therefore

of current vs. time, is beyond the scope of this discussion. We instead present the solution forthe specific case of cyclic voltammetry.

In cyclic voltammetry, one sweeps the potential of the working electrode at a specific

sweep rate (in volts / second), and measures the resulting current vs. time curve. Usually the

sweep is reversed at a specific switching potential, hence the name cyclic voltammetry. Since

the sweep rate is constant and the initial and switching potentials are known, one can easily

convert time to potential, and the usual protocol is to record current vs. applied potential. Figure

3 illustrates these concepts. The resulting current vs. applied potential curve (a cyclic

voltammogram) is predicted for an ideal, reversible system to have the shape shown on the far

right in Figure 3. The peak current ip in this voltammogram is given by;

ip = (2.69 105) n

32AD

12

12C

where ip is the peak current (in amperes), n is the number of electrons passed per molecule of

analyte oxidized or reduced, A is the electrode area (in cm2), D is the diffusion coefficient of

analyte (in cm2/sec), is the potential sweep rate (in volts/sec), and C is the concentration of

analyte in bulk solution (in moles/cm3). The midpoint potential of the two peaks in the

voltammogram is given by:

Emidpo int =(Ep,anodic +Ep,cathodic)

2=E 0 +

RT

nFln

DR1

2

DO1

2


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where Eo' is the redox potential, and DO and DRare the diffusion coefficients for the oxidized and

reduced halves of that couple. It is frequently reasonable to assume that DO and DR are nearly

equal, and in such a case the midpoint potential is very nearly equal to the redox potential.

Finally, the separation between the two peaks of the voltammogram is given by:

Ep = Ep,anodic Ep,cathodic = 2.3RT

nF=

59

nmV (at 298 K)

Hence, depending on what is already known about a given system, one could determine the

concentration, the diffusion coefficient, the number of electrons per molecule of analyte oxidized

or reduced, and/or the redox potential for the analyte, all from a single experiment.

Figure 3. Cyclic Voltammetry. Left; Eappl vs. time. Center; current vs. time. Right; current vs. Eappl.

Figure 4. Schematic diagram of an

References

1. Van Benschoten, J. J.; Lewis, J. Y.; Heineman,W. R.; Roston, D. A.; Kissinger, P. T., J.

Chem. Ed.1983, 60, 772.

2. Milner, D. J.; Rice, J. R.; Riggin, R. M.;Kissinger, P. T.,Anal. Chem. 1981, 53, 2258.

3. D.A. Skoog, F.J. Holler, T.A. Nieman,Principles of Instrumental AnalysisBrooks/Cole Publishing Co., 5

thedition, 1997,

cha ter 25.


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Cyclic Voltammetry Experiment

Materials

Glassware:

Miscellaneous volumetric flasks

One 10-mL volumetric pipetOne two-piece glass CV cell

One glassy carbon working electrode

One platinum working electrode

One platinum auxiliary electrode

One SCE reference electrode

Reagents:

Potassium dihydrogen phosphate (KH2PO4)

Potassium monohydrogen phosphate (K2HPO4)Concentrated sulfuric acid (H2SO4, 18 M)

Potassium ferrocyanide, trihydrate (422.41g/mol)

Methyl viologen dichloride, hydrate (257.17g/mol)

Nafion ion-exchange polymer (20 wt. % solution)

Procedure

Part 1: Reduction of Protons

1. Familiarize yourself with the four electrodes required for this experiment. The electrodesare stored in test tubes in a small wooden rack inside the faraday cage (the box whichhouses the cell):

* The reference electrode is a saturated calomel electrode or SCE and is readilyidentified as the only electrode that contains solution. This electrode is fragile and

contains liquid mercury so handle it very carefully. Keep this electrode upright at all

times (i.e., do not invert it), and make sure the electrode is always filled with a

saturated KCl solution.

* The auxiliary electrode consists of a coiled platinum wire sealed in a glass tube. It isreadily identified by its yellow and red wires.

* The platinum working electrode consists of a shiny platinum disk sealed in a glasstube. It is readily identified by its mirror finish and interior steel mesh.

* The glassy carbon working electrode consists of a shiny black carbon disk sealedinto a long translucent plastic tube. This electrode is readily identified by the

stainless steel rod protruding from the top.

2. Label a clean 25-mL volumetric flask as Dilute H2SO4. In the hood, use a Pasteur pipetto add 20 drops of concentrated sulfuric acid to the flask (CAUTION: Concentrated

sulfuric acid causes painful burns on contact with skin and instantly ruins clothing. Do

not splatter the liquid and dispose of your pipet carefully.) Dilute to the mark with

distilled water. The pH of this solution will be approximately zero.

3.

Label a clean 25-mL volumetric flask as H2O / 50 mM KH2PO4 / 50 mM K2HPO4 andfill it accordingly (KH2PO4 = 136.08 g/mol and K2HPO4 = 174.17 g/mol).

4. If it is not already done, clamp the upper half of the electrochemical cell onto the ringstand inside the faraday cage. Then, pour all of the dilute sulfuric acid you prepared in

step 2 into the bottom half of the electrochemical cell and attach it to the upper half with

rubber bands.

5. Ask your AI to help you accomplish the following tasks (in order):


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* Polish both working electrodes.* Fit the cell with the (freshly polished) platinum working electrode, the auxiliary

electrode, and the reference electrode, making sure that all three electrodes are

submerged, but not touching the bottom.

* Fit the cell with a bubbler(in this case a short Pasteur pipet) and plug the remainingthreaded adapter with the fitting provided.

* Attach the gas lines to the cell (one for the bubblerand one for the blanket).6. Begin sparging the solution: open the small round black-handled valves at both gas

regulators on the ring stand just to the left of thefaraday cage. Select nitrogen gas with

the upper three-way stopcock and select the bubbler with the lower three-way stopcock.

You should see a steady stream of bubbles exiting the tip of the bubbler.

7. Sparge the solution with nitrogen for approximately 5 minutes before going on to the nextstep. While you are waiting:

* Ensure that the CELL ENABLE switch on the potentiostat(the large rectangular unitbetween the faraday cage and the computer) is off.

* Carefully connect the electrodes to the external cell box in the faraday cage.(IMPORTANT: Never make or break connections to the cell while it is enabled.)

Note that the CNTR connection is for the auxiliary electrode. Have ONLY the

electrodes that are being used connected to the external cell box.

* Turn on the potentiostatthe power switch is located just above the bench top belowthe right side of the front panel.

* Log on to the PC and run the Cyclic Voltammetry program. An icon should bevisible on the desktop. If the program hangs up, press ALT+TAB to return to

Windows. Then, end the task and reboot the computer.

* Configure the program: press a key or click the mouse to exit the white informationscreen. Enter the Setup menu and then click Edita long list of parameters will

appear on the screen. Many of the values are irrelevant for this experiment, however

use the mouse to click in the appropriate fields as necessary to set the following

conditions:

Purge Time (PT) = 0 (pass will appear when you enter the zero)

Equil. Time (ET) = 15 s

Scan Rate (SR) = 100 mV/s

Initial. Pot. (IP) = 0.600 V

Vertex 1 Pot. (V1) = 0.400 V (the first switching potential)Vertex Delay (VD) = pass

Vertex 2 Pot. (V2) = pass (the secondswitching potential)

Final Pot. (FP) = 0.600 V

Ref. Elec. (RE) = SCE

When you have finished, click in the area below the parameter list to return to the

Setup menu.


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8. Divert the flow of nitrogen from the bubbler to the blanket by rotating the lower three-

way stopcock on the ring stand to the left of the faraday cage.

9. Examine the working electrode and gently tap it with your finger to release any bubbleswhich may have become trapped underneath it. Close (but do not latch) the faraday cage.

10.Enable the cell by pressing the CELL ENABLE button on the potentiostat.11.Click the Run menu item in the program. After a 15-second equilibration period, the

program will initiate and record the voltammogram. When it is finished, press the CELL

ENABLE button to disable the cell. (IMPORTANT: The cell must be enabled during a

run but should remain disabled at all other times.)

12.Click File >> Save data. Enter a unique (8-character max) filename and use the initials ofyour group members for the extension. For example: profdata.ljk.

13.Print a copy of this voltammogram (with a list of the relevant parameters) by clickingFile >> Print/plot data >> Graph. THE CV PROGRAM IS NOT ENTIRELY

COMPATIBLE WITH THE HP LASERJET 4: after the print job is completed, make

sure the printer has no other jobs in the queuethen take the printer offline (press the On

Line button) and press Shift + Reset. DO THIS AFTER EVERY PRINT JOB FROM

THE CV PROGRAM.

14.Determine the negative limit: examine the voltammogram on the printoutyou shouldsee an initially flat curve that rises steeply to a sharp point at the switching potential (V1).Record the maximum current in your laboratory notebook. We will take the negative

limit to be the potential at the extrapolated intersection of the linear portions of the

initially flat region and the first steep rise. To determine this potential graphically, use a

ruler and pencil to extend a straight line from left to right along a linear portion of the

initially flat curve. Similarly, extend a straight line down from a linear portion of the first

steep rise in the voltammogram until it crosses the first line. The negative limit is the

potential at the intersection of the lines and should be near the bottom of the first steeply

rising portion of the voltammogram.

15.Stir the solution in the cell by briefly diverting the flow of nitrogen from the blanket tothe bubbler with the lower three-way stopcock. After a few seconds, return the flow tothe blanket and check for bubbles under the working electrode as before.

16.Click Setup >> Edit and set the Vertex Delay (VD) to 10 s. Open the door to the faradaycage, enable the cell, and click Run to initiate a second voltammogram. When you reach

the vertex delay, crouch down in front of the faraday cage and look up at the surface of

the platinum working electrode. Look closely and record your observations in your

The characteristics of the triangular waveform applied to the cell are determined by the SR,

IP, V1, VD, and V2 parameters. In this case, we have programmed a simple triangle to begin

at 0.600 V, sweep to 0.400 V, and return to 0.600 V all at 100 mV/s. Later on, you will learn

how to use the V2 parameter to make a double triangle.


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laboratory notebook. If necessary, repeat steps 15 and 16 until you are sure that you see

something going on in there. Do not save or print this voltammogram.

17.Disable the cell and reset the Vertex Delay (VD) to zero.18.Divert the flow of nitrogen from the blanket to the bubbler by rotating the lower three-

way stopcock on the ring stand to the left of the faraday cage.

19.Disconnect the platinum working electrode and carefully remove it from the cell. Installand connect the glassy carbon working electrode.

20.A small amount of air will be introduced into the cell as result of the previous step, socontinue to bubble the solution with nitrogen for about 30 seconds. Then, return the flow

of nitrogen to the blanket.

21.Determine the negative limit associated with the glassy carbon electrode in this medium:collect voltammograms as described above decreasing the value of V1 (i.e., making it

more negative) until you pass the negative limit. Do not be deceived by small rises in

currentyou will know when you have passed the negative limit when the current rises

sharply to a level which is comparable to the one you recorded at step 14.IMPORTANT: Always stir the solution between voltammograms by briefly diverting the

flow of nitrogen to the bubbler and then returning it to the blanket. Save and print only

relevant data files.

22.Change the solution: carefully disconnect and remove all the electrodes from the cell.Rinse each electrode with distilled water and blot it dry with a paper towel. Remove the

bottom half of the cell and rinse the contents down the drain with distilled water.

Remove the upper half of the cell from its clamp and rinse it with distilled water. Dry

both cell pieces and return the upper half to the clamp. Fill the bottom half of the cell

with the phosphate buffer you prepared at step 3 and install the electrodes, bubbler, plug,

and tubing as before.

23.Determine the negative limits for both working electrodes in the phosphate buffer asdescribed above, starting with the platinum working electrode. Save and print only

relevant data files. When you are finished, do not disassemble the cell (i.e., leave the

glassy carbon electrode in the phosphate buffer and go on to part 2).

Part 2: Reduction of Oxygen

24.Set IP to 0.600 V and make V1 equal to the last negative limit you determined in step 23.Remember that the negative limit should be near the bottom of the first steeply risingportion of the voltammogramhence, this V1 should be more positive than the one used

to determine the negative limit (i.e., the current at V1 should not be overwhelming).

25.Collect and save one voltammogram but do not print it.26.Bubble the solution with air for about 2 minutes using the three-way stopcocks, then

divert the flow of air to the blanket.

27.Collect and save one voltammogram but do not print it.


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28.Bubble the solution with nitrogen for about 5 minutes using the three-way stopcocks,then divert the flow to the blanket.

29.Collect and save one voltammogram but do not print it.30.Click Graph >> Overlay and then select the data file you saved in step 25. Click Overlay

again and select the data file you saved in step 27. Print the overlaid data files by

clicking Main >> File >> Print/plot data >> Graph. Note that this graph will not have a

list of experimental parameters on it.

31.Do not prepare solutions for part 3 in advance. Rather, stop here (at the end of the firstday), turn off the gases and the instrument, clean your bench space (including the balance

area), and use any remaining time to begin the preparation of your report. In addition,

reboot the computer.

Part 3: Redox Chemistry of Methyl Viologen (MV)

32.Label a clean 100-mL volumetric flask as H2O / 50 mM KH2PO4 / 50 mM K2HPO4 andfill it accordingly (KH2PO4 = 136.08

g/mol; K2HPO4 = 174.17

g/mol).

33.Label a clean 25-mL volumetric flask as 5 mM MV in phosphate buffer and fill itaccordingly (i.e., use the solution from step 32 as diluent).

34.Clamp the top half of the cell in the faraday cage, fill the bottom half of the cell with thesolution you prepared in step 33, and attach it to the top half. Assemble the remaining

elements of the cell as before using a freshly polished glassy carbon working electrode.

Sparge the solution with nitrogen for 5 minutes and then divert the flow of gas to the

blanket. Check for bubbles.

35.In the Cyclic Voltammetry program set all the parameters listed in step 7 changing IPto +0.200 V, V1 to 0.950 V, FP to +0.200 V, and SR to 50 mV/s.

36.Collect, save, and print one voltammogram, remembering to enable and then disable thecell.

37.Stir the solution as described above (i.e., by bubbling).38.Repeat steps 35 through 37 with scan rates of 100, 250, 500, and 1000 mV/s.39.When you have collected the last voltammogram, use the Graph menu to overlay the

voltammograms you recorded at 500, 250, 100, and 50 mV/s onto the one you recorded at

1000 mV/s. Print this graph.

40.Set SR back to 50 mV/s and change V1 to 1.250 V.41.Collect, save, and print one voltammogram, remembering to first enable and then disable

the cell.

42.Return V1 to 0.950 V and set SR to 100 mV/s.43.Collect, save, and print one voltammogram, remembering to first enable and then disable

the cell.


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44.Carefully remove the cell bottom and take it over to the bench top. Pipet exactly 10.00mL of the solution inside into a clean, labeled 25-mL volumetric flask. Dilute to the

mark with the phosphate buffer you prepared in step 32.

45.Empty the remaining solution into the aqueous waste and rinse and dry the cell bottom.Disconnect the electrodes from the external cell box and disassemble the rest of the cell.

Rinse and dry each component (including the cell top).

46.Pour the solution you prepared in step 44 into the cell bottom and reassemble the cell.47.Repeat steps 43 through 46 two more times and then repeat step 43 (for a total of four

voltammogramsone at 5.0 mM and three at successively more dilute concentrations).

Finally, use the Graph menu to overlay all the voltammograms recorded for the different

concentrations of MV onto one graph and print it.

48.Dispose of any remaining MV solution in the aqueous waste, then disassemble and cleanthe cell.

Part 4: Polymer-Coated Electrodes

49.Label a clean 25-mL volumetric flask as 1.0 mM MV and 1.0 mM FC in phosphatebuffer and fill it accordingly (FC = ferrocyanide).

50.Fill the bottom half of the cell with this solution and assemble the rest of the cell asbefore (with the glassy carbon working electrode).

51.Sparge the solution with nitrogen for about 5 minutes. While you are waiting, configurethe program with the parameters listed in step 7 and the following:

IP = 0.400 V

V1 = 0.900 VV2 = 0.500 V

52.Divert the flow of nitrogen to the blanket, enable the cell, and then collect, save, and printone voltammogram.

53.Divert the flow of nitrogen to the bubbler and then disconnect and remove the glassycarbon working electrode.

54.Coat the working electrode with a Nafion film: rinse the electrode with distilled water,blot it dry with a Kimwipe

, and then clamp it upside down on a small ring stand. Adjust

the angle of the electrode in the clamp until the tip is level. With the aid of a Pasteur

pipet, place a single drop of the Nafion solution onto the working electrode surface and

allow it to air dry. If you prefer, you may use a gentle stream of compressed air from the

house lines to speed the drying process. Make sure that the film is really dry before

proceeding, because it will wash off in the cell if it is not.

55.The following three steps involve timing: insert the filmed electrode into the solution,bubble with nitrogen for an additional 30 seconds, divert the flow of nitrogen to the

blanket, and then collect and save one voltammogram (but do not print it).

These parameters instruct the potentiostat to apply a double triangle

to the cell, which allows one to examine mixtures containing both

reducible and oxidizable compounds in one scan.


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56.Stir the solution by means of bubbling for an additional 3 minutes and then collect andsave one voltammogram (but do not print it).

57.Repeat step 56.58.Use the Graph menu to overlay the voltammograms acquired at 0, 3, and 6 minutes and

print the graph.

59.Remove the filmed electrode from the solution, rinse it lightly with a little distilled water,and then carefully blot the body of the electrode dry with a Kimwipe. Do not attempt to

dry the filmed surfacethe film must remain in place for the next step.

60.Disassemble the cell and clean and dry each component (dispose of the MV/FC solutionin the aqueous waste). Then, refill the bottom half of the cell with the phosphate buffer

you prepared in step 32 and reassemble the cell with all the components exceptthe filmed

electrode.

61.Sparge the solution with nitrogen for about 5 minutes. Then, insert the filmed electrodeand sparge for an additional 30 seconds.

62.Divert the flow of nitrogen to the blanket, enable the cell, and collect and save onevoltammogram (but do not print it).

63.Disconnect and remove the working electrode. Wipe the tip with a Kimwipe to removethe film and then polish it as before.

64.Reinsert the freshly polished electrode, sparge the solution for about 30 seconds, divertthe flow to the blanket, enable the cell, and then collect and save one voltammogram (but

do not print it).

65.Use the Graph menu to overlay the two voltammograms acquired in the phosphate buffer(at steps 62 and 64).

66.Disassemble and clean the cell. The phosphate buffer is not contaminated and may berinsed down the drain with water.

Data Analysis

67.Use the Graph menu in the program to overlay the four best negative limitvoltammograms from part 1 and print the graph. Create a table listing the negative limits

you determined from the individual graphs, but include only the overlay graph in your

report.

68.Given that the Ka for dihydrogen phosphate is 6.2 108, calculate the pH of thephosphate buffer you prepared in step 3.

69.Write the reaction(s) which are happening at the negative limit.70.Answer the following:

* Why is the negative limit in a given medium always more positive for platinum thanit is for glassy carbon?

* Why is the negative limit for a given electrode always more positive in the dilute acidthan it is in the phosphate buffer?


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16/17

78.Given that the glassy carbon electrode was constructed from a 3-mm-diameter glassycarbon rod, use the slope of the line above to calculate the diffusion coefficient of methyl

viologen. Refer to the manual and show your work.

79.Using Microsoft Excel (or an equivalent program) create a single XY (Scatter) graphanalogous to that of step 76 for peak current and scan rate (at constant concentration).

Specify the chart type with markers only and use Excel to perform linear regression on

the data. Display the equation of the line with the correlation coefficient on the graph.

80.Use the slope of the line above to calculate the diffusion coefficient of methyl viologen.Show your work.

81.Compare the diffusion coefficients determined by the two methods above. State whichone you think is likely to be more accurate and why. What is the chief benefit of the

scan rate approach over the concentration approach?

82.For part 4, include all three graphs (one individual and two overlay) in your report.83.Examine the voltammogram you acquired at the bare electrode (step 52) and write all the

reactions occurring in this voltammogram. On the graph itself, assign each peak to areaction.

84.Compare the voltammograms you acquired at the filmed electrode (step 58) to the oneyou acquired at the bare electrode and account for the differences. In particular:

* Focus on the voltammetric behavior of methyl viologen at the bare electrode asopposed to that at the filmed electrode. Are there any differences? If so, what causes

these differences?

* For the filmed electrode, why does the concentration of methyl viologen appear toincrease as time goes on?

85.Account for the appearance of the voltammograms you collected for the filmed electrodein pure phosphate buffer (step 65).


17/17

Cyclic Voltammetry Report Grade Sheet

Section Points

Qualifier 25

Introduction 10

Experimental 10

Results and Discussion

Part 1: Reduction of Protons

Overlay graph 5

Buffer pH, reactions at negative limit 5

Dependence of () limit on electrode material 5

Dependence of () limit on solution 5

Part 2: Reduction of Oxygen

Overlay graph 5

Peak potentials 5Reversibility, importance of removing oxygen 5

Part 3: Redox Chemistry of Methyl Viologen

Data table 5

Graph for peak current and concentration, calculation of D 5

Graph for peak current and scan rate, calculation of D 5

Part 4: Polymer-Coated Electrodes

One individual and two overlay graphs 5

Reactions occurring 5

Evaluation of bare/filmed electrode in analyte solution 5

Evaluation of filmed electrode in buffer solution 5

Conclusion 5

Literature references 5

Documents

Information about CV